1. Field of the Invention
Embodiments of the invention generally relate to apparatus and methods for vaporizing liquid precursors. In particular, the apparatus and methods for vaporizing liquid precursors are drawn to bubbler nozzle structure and method for using.
2. Description of the Related Art
New advancements in semiconductor processing require improved delivery of liquid precursors to a processing chamber for using in multiple processes including material deposition processes. The liquid precursors are preferably applied in a vapor form for efficient use of the precursors and for effective control of the formation of deposited materials on a substrate in the processing chamber.
There are five accepted technologies that supply a liquid precursor vapor to a processing chamber. One process supplies the liquid precursor to a processing chamber in a liquid form with the flow rate controlled by liquid mass flow controller (LMFC) and then the precursor is evaporated by a vaporizer at the point of use. A second process involves a liquid precursor being evaporated by heating and the resulting vapor is supplied to a chamber with the flow rate controlled by a mass flow controller (MFC). A third process involves supplying a vapor of a precursor by flowing a carrier gas over the surface of the precursor contained in a canister and carrying the resultant precursor vapor out of the canister and subsequently to the process tool. In this process the carrier gas flow is never submerged in the precursor liquid. A fourth process provides a vapor of a liquid precursor from a canister to a chamber by vacuum draw system. Finally, a bubbling method may be used to vaporize the liquid precursor and deliver the resulting vapor to a processing chamber.
The bubbling method introduces the carrier gas into the precursor liquid so that a number of carrier gas bubbles are introduced into the precursor liquid. The carrier gas bubbles rise to the surface of the precursor and become saturated more or less close to the equilibrium vapor pressure of the precursor at the temperature of the canister. It may be appreciated by one skilled in the art that the efficiency of the bubbling process is effected by the size and rate of bubbles introduced into the precursor liquid as well as the height of the precursor liquid above the point of entry of the bubbles.
Precursors that are easily decomposed by heating cannot be supplied by the first two methods mentioned above. The subsequent two methods cannot be used for supplying a large amount of precursor without heating and it has been difficult to effectively control the resulting flow rate of the heated liquid precursor vapor. The fifth process, termed the bubbling method does solve the difficulties in the first four processes; however, it has been observed that the bubbling method has difficulty maintaining a constant concentration and constant temperature during supply of the vaporized precursor.
Additionally, prior bubbling systems have been observed to have less than satisfactory bubbling results and less than satisfactory flow rates. For example, prior bubbling systems have difficulty providing a consistent and large amount of flow rate of precursor for use in the solar cell field manufacturing.
In one embodiment of a known bubbling system, a carrier gas is introduced into the liquid precursor through a dip-tube in the bubbler and removes evaporated vapor gas of the precursor from the bubbler. The mixture of carrier gas and precursor is supplied to a processing chamber, such as a chemical vapor deposition (CVD) chamber. In such as bubbling system, it is important to supply the precursor stably to the processing chamber at a high and efficient flow rate, and also to supply precursor without condensation at the supply piping line, and to ensure that all of the precursor that exits the canister in the carrier gas is fully vaporized and not in the form of droplets. The change of precursor concentration and condensation or existence of droplets of the precursor will affect the reproducibility and repeatability in a film forming process (CVD) including defects in the film uniformities of atomic component and/or thickness of the deposited films. Especially, condensation or the formation of droplets at the canister exit of the precursor in the supply piping has been observed to have a remarkable concentration change.
FIG. 1 shows the prior bubbling system 100 having a hot water bath to maintain constant liquid temperature. In FIG. 1, a bubbler nozzle 102 is connected to the end of a DIP tube 104 and generates bubbles 106 by flow of a gas into a volume of liquid precursor 108 in a container 120. Inert gas is introduced into the DIP tube using an inlet valve 110 (head valve), and vaporized precursor formed from the bubbles exits the container 120 via an outlet valve 112 (DIP valve). The prior bubbling system 102 uses prior bubbler nozzles 102, of which some embodiments 200, 220 are shown in FIGS. 2A and 2B having the respective gas inlets 210, 230, and the respective nozzle outlets 212, 232.
In the prior bubbling system 100, a water hot bath 130 is widely used to maintain the liquid precursor 132 at a constant temperature. The liquid precursor 108 in the container 120 (bubbler) loses heat through evaporation and the hot water bath 130 supplies the necessary heat to balance the heat that is lost by bubbling of the precursor liquid 108. The hot water bath 130 consists of a tank 134, a thermocouple 136, a sensor cable 138 of thermocouple 136, and a controller 140 that can control the liquid temperature by using a temperature signal from thermocouple 136. The hot water bath 130 is coupled to a water jacket heater surrounding the bubbler, or container 120 through lines 116 and 118, which lines are piping for circulating the hot water from the tank 134 to the water jacket heater 114.
The prior liquid control system has the following problems. It is difficult to maintain a constant liquid temperature because the hot water heated by water bath tank has to be circulated in the water jacket heater through piping line. It is difficult to heat the bottom of the bubbler and thus, heat the liquid in the bubbler efficiently. The system has a slow heat adjustment response time when dealing with a decrease of liquid temperature by evaporation of liquid precursor because there is a distance from heater bath tank and liquid precursor that has to be heated. This prior heating system requires extra costs and additional equipment including the water bath and hot water piping lines. Such a system requires additional maintenance to change the water in the water bath tank and cleaning the water bath tank. The water in the water bath tank is also a very dangerous liquid as the precursor reacts with water violently. Additionally, in the some cases, the heating system using water bath cannot be used at clean room in the semiconductor factory. Additionally, It is difficult to supply a precursor stably at the high flow rate without the use of a high flow rate of carrier gas and/or maintain high liquid temperature.
Further, there are three problems about bubbling supply as prior techniques. The first problem is the undesired time required to obtain constant concentration of liquid vapor at the beginning of supply. This problem causes an increase of waste of the precursor because the unstable concentration of precursor affects the property of the semiconductor device and the unstable concentration cannot be used for deposition purposes. A sudden decrease of liquid temperature due to evaporation heat also occurs when the bubbling supply is started, and if the heater cannot respond to the change, then the temperature must be increased to a higher than set value and overheat the heater.
The second problem is the fluctuating concentration of the precursor during bubbling. This problem causes the property of the product, such as semiconductor device or photovoltaic cell, to be less than desired as the unstable chemical vapor concentration affects thickness and uniformity of the substrate. The problem might also occur by the mist generation and/or undesired bubble size. Mist generation may be caused by pulsation (instability of surface of liquid precursor caused by waves or splashing) at the liquid precursor surface. The vapor concentration is easily fluctuated by generated mists as the mist precursor material is entrained in the exiting precursor flow and the mist forms irregularly and unpredictably.
There are two possible reasons for pulsation. The first reason is the bursting of bubbles having large sizes at the surface of the liquid precursor cause an increase in pulsation and an increase in flow of liquid precursor to the surface. The second reason is that the generated bubbles are not delocalized uniformly in the liquid sufficiently causing focused evaporation points in the liquid surface and partial bursting at surface of liquid. FIG. 6A shows the bubble generation from the tube edge. The bubbles generated from this nozzle are obviously large and the pulsation of the liquid surface has been observed to be heavy. FIG. 6B indicates the bubble generation from one common solution. The nozzle in FIG. 2B can reduce the size of bubbles well in comparison with the nozzle FIG. 2A. But the pulsation of the surface is not decreased because of the generating the bubble pathway made by rising bubbles (bursting bubbles at same points of the liquid surface).
The third problem occurs at low liquid levels. The concentration of the precursor rises/drops rapidly when the liquid level is close to empty in the bubbler. Therefore, the liquid in the bubbler cannot be effectively used. When the liquid level is closed to bubbler nozzle, the generated mists sometime increase the concentration, or the concentration decreased by lack of evaporation.
Therefore, there is a need for a process and apparatus for effectively bubbling a liquid precursor and delivering the precursor to a processing chamber.